1. Field of the Invention
The present invention relates to a revolution speed control apparatus for an internal combustion engine. In particular, the invention relates to a revolution speed control apparatus for an internal combustion engine that employs a variable valve drive device incorporating a three-dimensional cam whose profile continuously changes in a direction of a rotational axis of the cam.
2. Description of the Related Art
A known variable valve drive device for intake or exhaust valves of an internal combustion engine switches between a valve lift set for low revolution speeds of the engine and a valve lift set for high revolution speeds. This is performed by hydraulically switching between a low speed cam and a high speed cam, as described in, for example, Japanese Patent Application Laid-Open No. Hei 1-19131.
This technology employs countermeasures against reductions in the responsiveness of the switching from the low speed cam to the high speed cam caused by low hydraulic fluid viscosity at low temperatures. That is, during a low temperature condition, the variable valve drive device maintains operation of the low speed cam, and stops the fuel supply to the internal combustion engine if the engine revolution speed becomes high.
However, if the temperature is not low, the variable valve drive device controls the switching between the low speed cam and the high speed cam merely by controlling the cam switching fluid pressure, and does not check whether the cam is actually switched. Therefore, if the low speed cam is not switched to the high speed cam, for any reason, at the time of a high engine revolution speed, the variable valve drive device is not able to perform a fail-safe operation, such as a fuel cut operation, or the like.
In addition to the above-described system for switching between the low and high speed cams, another variable valve drive device is known which employs a three-dimensional cam whose cam profile continuously changes in the direction of the rotational axis of the cam, i.e., to adjust the valve lift. In some systems employing such three-dimensional cams, the valve lift adjustment involves, as parameters, not only the revolution speed of the internal combustion engine, but also engine loads including the intake pressure, the amount of intake air, the amount of fuel, and the like. In such a system, detection of an actual revolution speed is not sufficient to determine a single cam profile to be engaged. Therefore, if the control is solely based on detection of the engine revolution speed, it is impossible to determine a valve lift and set an allowable revolution speed. Hence, a mere application of the conventional technology, for switching between a low speed cam and a high speed cam, to a variable valve drive device employing a three-dimensional cam results in difficulty to establish a fail-safe system.